Litecoin is a fork of Bitcoin as of 2011. It is often refered to as the "Silver to Bitcoin's Gold", but it primarily differs in the following key ways:
- Shorter blocktime of 2.5 minutes vs 10 minutes (faster transactions)
- Maximum of 84 million coins vs 21 million Bitcoins
- Uses an algorithm that's resistant to centralized ASIC mining
The Litecoin team also aggresively under the guidance of it's founder Charlie Lee are developing imporvements that often get rolled back into Bitcoin. Examples are segwit (to help with blocksize) and Lightning Network (so called off-chain fast payments).